Ch. XVII] fossils of the UPPEK CRETxiCEOUS BEDS. 



249 



are dispersed indifferently througli the soft clialk and hard flint, and some 

 of the flinty nodules owe their irregular forms to inclosed sponges, such as 

 fig. 285 a, where the hollows in the exterior are caused by the branches 

 of a sponge, seen on breaking open the flint (fig. 285 b). 



Fig. 285. 



Up 



branching sponge in a flint, from the white chalk. 

 From the collection of Mr. Bowerbank. 



Fig. 2S6. 



Siphonia pyri- 

 formis. 



Chalk marl. 



The remains of fishes of the Upper Cretaceous formations consist 

 chiefly of teeth of the shark family, of genera in part common to the 



Fig. 287. 



Palatal tooth of 



Ptychodics decurrens. 



Lower Avhite chalk. 



Maidstone. 



CeRtracion Phillippi; recent. 

 Port Jackson. Buckland, Bridgewater Treatise, pi. 27, d. 



tertiary, and partly distinct. To the latter belongs the genus Ptijchodus 

 (fig. 287), which is allied to the living Port Jackson Shark, Cestracion 



